(Codegen) Pthread platform forces nbPeType to 1

[antoine-morvan]

Spider Pthread platform does not support architecture definition with multiple PE types: the CPP code forces the value to 1. See: https://github.com/preesm/spider/blob/master/master/libspider/platforms/platform_pthread/platformPThread.cpp#L243

• Spider runtime should allow to set this value;
• Codegen in Preesm should generate code for setting appropriate values;